Linear transmitters are known. A negative feedback (e.g., a Cartesian feedback) transmitter constitutes one such known transmitter. When properly adjusted, a negative feedback transmitter satisfactorily operates in a linear mode. That is, negative feedback linear transmitters require frequent feedback loop adjustments to maintain their linear performance. Such adjustments are typically made to the phase shift of, and to the level of the signal inputted to, the feedback loop incorporated within the linear transmitter. When these parameters are left unadjusted, changes in the operating or environmental conditions of the linear transmitter often result in undesired interference, or splatter, into frequencies substantially adjacent to the operating frequency of the linear transmitter. For example, a decrease in the supply voltage to the transmitter's amplifier stages, or operation of the transmitter at high ambient temperatures (e.g., 40 degrees Celsius), might result in decreased loop gain and undesired clipping of the transmitter's output signal, thereby producing adjacent channel splatter.
To insure proper adjustment of the feedback loop parameters, one prior an approach provides for training of the linear transmitter to periodically adjust the loop phase and gain. This approach is described in detail in U.S. Pat. No. 5,066,923, entitled "Linear Transmitter Training Method And Apparatus," and assigned to Motorola, Inc. Although this approach minimizes adjacent channel splatter produced by the linear transmitter during normal transmitter operation, under some conditions the training signal itself may produce undesired levels of adjacent channel splatter. For example, the splatter produced during training can cause off-channel interference in geographic areas of dense spectral utilization (i.e., closely adjacent frequencies) such as urban areas.
Therefore, a need exists for a method of reducing off-channel interference produced by the training of a linear transmitter.